In classic x-ray tubes used in radiography, free electrons must be made available so they can be accelerated by a high-voltage electric field, hit a target made of high-density, high-melting point metal (usually tungsten) attached to an electrode called an anode, and cause x-rays to be generated and emitted as a consequence of their rapid deceleration in the anodic target.
Such free electrons are produced by another electrode called a cathode (to which the negative pole of the high-voltage circuit is connected). Generally, electrons are freed from the cathode by thermal emission. To accomplish that, the cathode is usually in the form of a filament (also usually made out of tungsten) which is heated to glowing temperature through the passage of substantial electric current, called the filament current. In this way, the cathode (filament) simultaneously is associated with two different circuits, (i) the above-mentioned filament circuit, and (ii) the anodic circuit, across which the high voltage is applied for the electric field that accelerates the x-ray-yielding electrons.
The number of electrons emitted, and consequently the anodic current and the intensity of the x-ray beam that is generated, depends upon the temperature of the filament being elevated to a certain level by the electrical current. Therefore, the anodic current is a very steep function of the filament current. Consequently it is imperative that the filament current, and the operation of the filament circuit in general, be well controlled and regulated, in order to ensure a stable, consistent, and predictable anodic current and resultant x-ray intensity, or radiation dose rate.
One of the problems—indeed probably the most challenging to address—which must be resolved in order to implement such accurate regulation, occurs at the onset of the electron emission. At the quiescent state, if there is no current flowing through the filament, the filament is at an ambient temperature that is considerably lower than the filament temperature reached during emission of electrons and x-rays. Consequently, the filament electrical resistance is also much lower at an ambient temperature than during emission, since the electrical resistance in metals increases with an approximate linear dependence with the absolute temperature. As electrical power is applied, and the filament current begins to flow through the filament, its temperature and its resistance starts to increase, until a steady-state condition is reached where the amount of electrical power dissipated in the filament is in equilibrium with the thermal dissipation from the filament (which is also proportional to the temperature reached by the filament). Other second-order phenomena also affect this equilibrium condition, such as the power drain and temperature drop caused by the electrons of the anodic current being stripped away from the filament. Due to the thermal inertia of the filament, and the fact that its initial electrical resistance is low and so is the power it dissipates, normally it takes several hundreds of milliseconds for the x-ray tube to reach electrical equilibrium.
Consequently, if the filament of the x-ray tube is abruptly powered from ambient temperature, several tenths of a second may be required for the radiation output to rise to the desired, final level. This delay is undesirable especially with modern digital x-ray image receptors, which may require, or take advantage of, short exposure time, and consequently reduce the radiation dose to the patient.
In most modern x-ray source designs, where the filament circuit can be controlled and powered independently from the anodic circuit, during the quiescent state the filament is continuously powered with a moderate-intensity current (is “glowing”), that maintains the filament at an elevated temperature although the elevated temperature is less than the filament temperature achieved during emission. In this manner, the filament's electrical resistance is much higher than at ambient temperature, and it will respond much faster to a further rise of the applied electric power.
A further improvement, which is commonly adopted, is to boost the electrical power applied to the filament for a short time (e.g., a few hundredths of a second) before the application of the high voltage to the anodic circuit, in order to heat the filament to such a temperature that electronic current at the onset of the high-voltage corresponds substantially to the desired steady-state value that will settle within a few milliseconds. This is a called the preheating boost.
In order to accomplish a preheating boost, however, the preheating current or power to the filament usually needs to be accurately adjusted on an individual basis in each x-ray source. This individual adjustment is due to the very steep and critical dependence of the anodic current to an electrical current and temperature of the filament, as already mentioned, whereas minor physical and material differences between actual filaments and x-ray tubes (well within the constructive tolerances practically achievable) may lead to a significant difference among such onset anodic current.
Often, and especially in case of so called DC-supplied x-ray sources, the anodic current, and the filament power that controls it, is regulated through a feedback controlled loop. The feedback loop ensures that the anodic current ultimately settles to the target value. However, if the onset value is significantly different from the target (steady state) value, initially anodic current will be subject to large transitory fluctuations, such as shown in FIG. 1. Typically, such transitory fluctuations may last for several hundredths or even tenths of a second, which is a time frame incompatible with the short exposure time required with digital electronic image sensors, or even with “fast films”. In the extreme case, such transitory fluctuations may bring anodic current out of scale, that is, beyond the range permitted by electrical safety controls, and cause the system to abort emission.
Consequently, even if the value of the anodic current is ultimately regulated through a feedback loop (acting upon the filament power via a nested loop), it is still necessary to accurately adjust the value of the preheating filament power (or current, or voltage), by calibrating for each individual x-ray source. Such calibration is critical and easily subject to operator errors.
Furthermore, if the target anodic current and/or the anodic high voltage (the “technique factors”, as they are called in radiology) are not fixed to one value only (as is the case for most actual x-ray sources except most of those used for intraoral dental radiography) then such adjustment depends upon the specific technique factors selected for that emission. Such dependency is very direct for anodic current, but is affected also by the selected anodic high voltage. Consequently, even if a correction is applied to the preheating power to account for different technique factors, such correction may not operate exactly in the same manner, and equally well, in each individual unit.
In addition, such adjustment may not be stable as a result of changing environmental conditions, and may likely drift over the life span of the x-ray tube as a consequence of the filament aging. This problem has no known solution with the usual design in the current art, except performing regular calibration re-adjustments.
What is needed is a system and method for a dental x-ray device that automatically calibrates control parameters to the filament.